Our lab is studying the transport of cerium ions across the epithelium of the renal proximal convoluted tubule. During transport, the cerium precipitates out, presumably upon encountering phosphate ions, producing an electron dense, nondiffusable product within the lateral spaces between epithelial cells. This cerium precipitate also impregnates the basolateral membrane, resulting in twenty to thirty 3 nanometer punctate densities that occupy oval domains along the basolateral membrane. These "microdomains" are clearly visible with a conventional TEM and with the JEOL 1000 HVEM. The HVEM offers the ability to examine 0.5-1.0 micron thick sections that reveal the three-dimensional distribution of these microdomains along the basolateral membrane, especially when examined as stereo pairs. We are exploring the possibility of using high voltage tomography to ascertain whether the position of cerium microdomains along the lateral cell membrane correlates in three-dimensional space with that of contiguous structures. The two best candidates are dense apical tubules and the fenestrated smooth endoplasmic reticulum that forms plates against the lateral cell membrane. These two organelles may play an important role, either directly or indirectly, in transcytosis of cerium in the proximal renal tubule.